LONGITUDINAL MODE SEPARATION TUNING IN 633 NM HELIUM-NEON LASERS USING INDUCED CAVITY BIREFRINGENCE

The theory of laser cavity birefringence induced by insertion of two q uarter-wave plates into the cavity is developed to show how the interm ode frequency of a two-mode laser can be tuned through a free spectral range by relative rotation of the plates. Birefringence is demonstrat ed in a helium-neon laser at 633 nm using an open cavity having a tube with anti-reflection coated windows. The lock-in between modes when t he intermodal frequency approaches zero was investigated and found to be smallest when the tube was placed between the plates which were ali gned with axes parallel. Active frequency stabilization at a frequency separation of 50 MHz was demonstrated using the mode-balancing techni que.